Anodic treatment of zinc and zinc-base alloys



United States Patent 3,335,074 ANODIC TREATMENT OF ZINC AND ZINC-BASE ALLOYS Maurice Morgan Wright, Rossland, British Columbia, and

Hyman Guttman, Trail, British Columbia, Canada, assignors to The Consolidated Mining and Smelting Company of Canada Limited, Montreal, Quebec, Canada No Drawing. Filed July 13, 1964, Ser. No. 382,346 Claims priority, application Canada, May 14, 1964, 902,798 15 Claims. (Cl. 20456) The present invention relates to an anodizing bath and a process for producing adherent, corrosion and abrasion resistant coatings on surfaces composed of zinc and zincbase alloys.

According to United States Patent No. 3,011,958, a process is known for applying a corrosion and abrasion resistant coating to zinc and zinc-base alloy surfaces comprising the step of subjecting to electrolysis an aqueous solution which contains chromate, phosphate, and fluoride anions provided by compounds selected from chromic acid, phosphoric acid, hydrofluoric acid, and the ammonium salts thereof. The solution has a pH value within the range of from about pH 6 to about pH 8 and is substantially free from metal ions. The surface of at least one of the electrodes comprises zinc or a zinc-base alloy to be coated. Either alternating current or direct current can be used. With alternating current, the preferred current density is about 50-200 amperes per square foot of electrode surface per electrode. The voltage rises as the electrolysis proceeds, due to the increasing resistance of the coating as it is formed. Electrolysis is continued until the voltage rises to a levelling, or finishing, value of at least about 200 volts, usually from about 200 to about 250 volts.

Further, according to applicants copending application No. 329,941, filed -Dec. 12, 1963, it has been found that excellent coatings can be obtained on Zinc and zinc-base alloy surfaces by the electrolysis of an aqueous solution containing phosphate and chromate anions, and at least one'of the cations sodium and potassium, the phosphate anion calculated as P0 being within the range of from about 0.3 to about 3 moles per liter, the chromate anion calculated as CrO, being within the range of from about 0.1 to about 3 moles per liter, the said solution having a pH value within the range of from pH to pH 11.4. In this solution the chromate anion according to the copending application may be replaced at least in part by tungstate, molybdate or vanadate anion. According to this copending application the use of such a coating or anodizing solution allowed the electrolysis to be conducted at a voltage substantially below that necessary for the electrolytic process of United States Patent No. 3,011,958 and thus instead of a final voltage of 200 to 250 volts, a finishing voltage of only 35 to 110 is necessary.

Applicants have now found that an anodic bath oonsisting of an aqueous solution containing silicate anion calculated as SiO in an amount of from about 0.2. to about 2 moles per liter, chromate anion calculated as CrO in an amount from about 0.05 to 1.5 moles per liter and alkali metal cation selected from sodium and potassium, said solution having a pH from about 11 to 13, also produces excellent coatings, usually harder and tougher,

on zinc and zinc-base alloy surfaces by electrolysis of said aqueous solution using in said bath at least one electrode having exposed surfaces formed by zinc or a zinc-base alloy.

provided an anodizing bath consisting of an aqueous solution containing silicate anion calculated as SiO' in an amount of from about 0.2 to about 2 moles per liter, chromate anion calculated as CrO in an amount from about 0.05 to 1.5 moles per liter and alkali metal cation selected from sodium and potassium, said solution having a pH from about 11 to 13.

The present invention also provides an anodic process for coating zinc and zinc-base alloy surfaces which com prises electrolysing using AC current an aqueous solution containing silicate anion calculated as SiO= in an amount of from about 0.2 to about 2 moles per liter, chromate anion calculated as CrOL, in an amount from about 0.05 to 1.5 moles per liter and alkali metal cation selected from sodium and potassium, said solution having a pH from about 11 to 13, using at least one electrode having exposed surfaces formed by a metal selected from zinc and zinc-base alloys, until a coating is formed on said electrode.

The pH range of the aqueous solution, i.e. electrolyte of the anodizing bath, for satisfactory film formation is from about pH 11 to about pH 13, preferably from pH 11 to pH 12.5. The alkalinity of the solution may also be defined by specifying the Na O/SiO mole ratio, R. The Na O figure is obtained by deducting from the total Na O content of .the solution the amount of Na O associated with the other anions present, such as sodium chromate for chromate, sodium vanadate for vanadate and sodium molybdate for molybdate. Generally speaking, the value of R should not be less than 0.3 and not be greater than 1.0, and more preferably be in the range of 0.4 to 0.7.

The temperature of the bath should be maintained in the range of from 25 to 95 C, but it is preferred to maintain the temperature of the bath within the range of about C to C.

In order to produce satisfactory coatings the amount of chromate anion calculated as CrO, should be in the range from about 0.05 to about 1.5 moles per liter and more preferably the amount of chromate anion should be in the range of from 0.1 to 0.3 moles per liter. Similarly, for satisfactory coatings the silicate anion concentration should be in the range of 0.2 to 2.0 moles per liter calculated as SiO and more preferably in the range of 0.2 to 0.4 moles per liter. It is immaterial which of the alkali metal cations is present, both sodium and potassium and mixtures thereof serving equally Well.

Alternating current is used in the process of the present invention; direct current does not produce as satisfactory a film. With AC, both electrodes can and should be articles formed of or surfaced with zinc or a zinc base alloy.

In the electrolysis of the articles the initial current density should be in the range of from about 350 to about 1000 amps per square foot until the formation of a film is initiated and thereafter the current density should be reduced to a range of to about 300 amps per square foot of electrode surface per elect-rode and the electrolysis continued until the voltage rises to a terminating value According to the present invention therefore there is of from 55 to 250' volts. In a preferred embodiment of the present invention the initial current density is from 465 to 700 amps per square foot and the final current density is in the range 115 to 230 amps per square foot and the terminating voltage is from 100 to 200 volts.

Applicants have found that the chromate anion can be replaced in part or in whole by at least one anion selected from vanadate, molybd-ate, permanganate and tungstate anion.

In operation of the process of the present invention the surfaces of the articles to be anodized are first degreased and cleaned. The aqueous solution may be readily prepared by dissolving sodium or potassium salts in water and adjusting the pH to the desired value with an alkali metal hydroxide. Chromate anion can be conveniently added as chromic acid, and vanadate, permanganate, molybdate and tungstate anion as their alkali metal salts. The cleaned articles are then immersed as electrodes in the squeous solution as electrolyte and connected to a suitable power source supplying 60 cycle AC within the range of from to 250 volts. The electrolyte is agitated and maintained at a temperature of from 25 to 95 C. during the electrolysis. Normally, a fairly high current density up to 1000 amperes per square foot, is required to start the formation of the film or coating but once the formation of this film has been initiated, the current density can be reduced to 125 amps per square foot or lower and the coating will continue to grow. As electrolysis proceeds, the voltage rises to a value which may be as high as 250 volts. Electrolysis may be terminated at a lower voltage depending on the quality and type of coating desired. The coated articles are removed, Washed and dried.

Film formation is believed to take place in stages. In the first, or induction period, during which the voltage rises very slowly, a film, probably Zinc oxide, forms on the surface of the Work piece. This induction period is followed by a period in which the voltage rises very rapidly and in which there occurs the initial deposition of anion (SiO CrO V0 M00 MnO W0 Following after this second period, a sparking phenomenon occurs on the surface of the article being coated, further anion deposition takes place and the film takes on a hard, dense attractive finish.

With further increase in voltage, the sparks become larger and more localized. This point may be taken as the terminating voltage, and at this stage the process is complete. Raising the voltage beyond this terminating value can result in deterioration of the film.

There are many 'factors that affect the terminating voltage, such as electrolyte composition, temperature, and the type and condition of the surface being anodized. The terminating voltage will usually be within the range of 4 from about to about 250 volts. It is found that generally sparking commences in the range of 50 to 200 volts; under preferred conditions of electrolyte composition and temperature, sparking commences in the range of to 110 volts.

The time required to complete film formation varies with conditions but is usually of the order of 5 to 15 minutes.

In another aspect of the present invention in order to avoid the use of a high current density to initiate the coating of the article having a zinc or zinc-base alloy surface one can pretreat the surface of the article in order to facilitate film formation for example by applying a thin coat thereto using the process of United States Patent No. 3,011,958 as aforesaid, or by application thereto of a phosphate coating by spraying with or dipping in phosphate solutions such as those sold under the trade names Litho-form, Granodine and Bonderite. With such pretreatment the initial current density is for example within the range of 115 to 230 amps per square foot of electrode surface to be coated.

By the process of the present invention it is possible to apply to articles formed from zinc or zinc-base alloys, for example die-cast or rolled zinc panels, or articles electroplated or hot dipped in zinc or zinc-base alloys, a coating or film which improves the corrosion resistance and appearance of the article. The coating gives a hard, smooth matte finish in shades of green, grey or brown depending upon the electrolyte composition, for example, a sodium silicate-chromate electrolyte will give a green shade; a sodium silicate chromate-vanadate electrolyte will give a charcoal grey shade; and permanganate-containing electrolytes will give shades of brown. However, the coating can, if desired, be painted or lacquered. The coating is resistant to abrasion and corrosion by various media such as industrial atmospheres, salt water and detergent solutions. The process is efiiective and can be applied to surfaces of commercial grades of zinc and zinc alloys such as special high-grade zinc, and alloys used in rolling, extruding and die-casting operations.

The present invention will be further illustrated by way of the examples in Table 1, which illustrate the conditions under which satisfactory coatings are produced by the process of the present invention, and those in Table 2 which illustrate the conditions under which satisfactory coatings were produced by the process of the invention for three electrolytes using die-cast Zinc plates which had been given a preliminary treatment in an ammonium phosphate-chromate-fiuoride bath according to United States Patent No. 3,011,958 until a potential of 110 volts AC was reached; the articles were then rinsed and allowed to dry before being anodized in the electrolyte of the present invention.

TABLE 1 Electrolyte-Molar Composition Current Density,

amps/sq. foot Sparking Terminat- Example No. Voltage ing Volta e 810 OrOr- VOr MoOr- R-Factor Initiation Growth g 5 A series of tests in which the article to be anodized was first given a thin coating in an ammonium phosphatechromate-fluoride electrolyte and then anodized under the following conditions:

TABLE 2 System SSC SSCMn Electrolyte Moles/Liter: NagSiOafiHaO...

Terminating Voltage. Anodizing Time (minutes). 7. 7. 5 Temp. of Electrolyte C.) 60-62 71-74 84-87 2. A bath as claimed in claim 1 the range from 11 to 12.5.

3. A bath as claimed in claim 1 in which the silicate anion calculated as SiO is present in an amount from 0.2 to 0.4 mole per liter.

4. A bath as claimed in claim 1 in which the chromate anion calculated as CrO is present in an amount from 0.1 to 0.3 mole per liter.

5. An anodizing bath for coating zinc and zinc-base alloy surfaces consisting of an aqueous solution consisting essentially of silicate anion calculated as SiO in an amount from 0.2 to 0.4 mole per liter, chromate anion calculated as CrO in an amount of from 0.1 to 0.3 mole per liter, and cations of at least one alkali metal selected from the group consisting of sodium and potassium, said solution having a pH in the range from 11 to 12.5.

6. An anod-ic process for coating zinc and zinc-base alloy surfaces which comprises electrolysing using alternating current an aqueous solution containing silicate anion calculated as SiO in an amount of from about 0.2 to about 2 moles per liter, chromate anion calculated as CrO in an amount from about 0.05 to 1.5 moles per liter and cations of at least one alkali metal selected from the group consisting of sodium and potassium, said solution having a pH from about 11 to 13, using electrodes having exposed surfaces formed by a metal selected from zinc and zinc-base alloys until a coating is formed on said electrodes.

in which the pH is in SSOMoSodium silieate-chromate-molybdate; SSW-Sodium silicate-tungstate.

In an acetic acid accelerated salt spray corrosion test (ASTM Method B 287) the SSCV coating was best showing no deterioration until the twelfth day of the test. The SSC coating stood up for 10 days, the SSCMn coating for 9 days. By comparison, the ammonium phosphate-chro- 1 mate-fluoride film of United States Patent No. 3,011,958 stood up for 5 days.

Detergent resistance was determined by subjecting anodized panels to constant immersion in a 2.5 gm./l. solution of a standard household detergent, sold under the trade name Tide, maintained at a temperature of 185 F. The coatings were examined periodically and rated on the basis of leaching and discoloration, softening, adhesion and base metal corrosion. After days in test, the SSV coating had shown no deterioration and was rated as best. By comparison the SSCV, the SSC and SSCW were almost as satisfactory and were rated as good. The SSW, SSMo, SSCMo, SSMn and the SSCMn showed varying degrees of deterioration and relatively were rated as fair.

The use of the term mole in the tables refers to gram molecular weight.

What we claim as our invention is:

1. An anodizing bath for coating zinc and zinc-base alloy surfaces consisting of an aqueous solution consisting essentially of silicate anion calculated as SiO in an amount of from about 0.2 to about 2 moles per liter, chromate anion calculated as CrO in an amount from about 0.05 to 1.5 moles per liter, and cations of at least one alkali metal selected from the group consisting of sodium and potassium, said solution having a pH from about 11 to 13.

7. A process as claimed in claim 6 in which the temperature of the bath is from 25 C. to C.

8. A process as claimed in claim 6 in which the current density is from 35 0 to 1000 amps per square foot of electrode surface per electrode initially until the formation of a film is initiated and thereafter the current density is reduced to to 300 amps per square foot of electrode surface and the electrolysis continued with increasing voltage to a terminating value in the range of from 55 to 250 volts.

9. A process as claimed in claim 6 in which the electrolysis is continued for a period of from 5 to 15 minutes.

10. An anodic process for coating zinc and zinc-base alloy surfaces which comprises electrolysing using alternating current an aqueous solution containing silicate anion calculated as SiO in an amount from 0.2 to 0.4 mole per liter, chromate anion calculated as CrO in an amount from 0.1 to 0.3 mole per liter, and cations of at least one alkali metal selected from the group consisting of sodium and potassium, said solution having a pH in the range from 11 to 12.5, the temperature of the bath being maintained within the range from 25 C. to 95 C. using in said solution electrodes having exposed surfaces formed by a metal selected from zinc and zinc-base alloys, conducting said electrolysis initially at a current density from about 350 to 1000 amps per square foot of electrode surface per electrode until the formation of a film is initiated, thereafter reducing the current density to within the range of about 100 to about 300 amps per square foot of electrode surface and continuing said electrolysis with increasing voltage to a terminating value within the range of from 55 to 250 volts.

11. A process as claimed in claim 10 in which the initial current density is 465 to 700 amps per square foot of electrode surface, the current density is subsequently reduced to a range of from 115 to 230 amps per square foot of electrode surface and the terminating voltage is within the range of 100 to 200 volts.

12. A process as claimed in claim 10 in which the temperature is from 60 to 80 C.

13. An article of manufacture having surfaces formed from a metal selected from zinc and zinc-base alloys coated with a film produced by the process of claim 6.

14. An anodizing bath for coating zinc and zinc-base alloy surfaces consisting of an aqueous solution consisting essentially of silicate anion calculated as SiO in an amount of from 0.2 to about 2 moles per liter, and at least one member selected from the group consisting of chromate anion calculated as CrO vanadate, permanganate, molybdate and tungstate anion and mixtures thereof in an amount from about 0.05 to 1.5 moles per liter, and cations of at least one alkali metal selected from the group consisting of sodium and potassium, said solution having a pH of from about 11 to 13.

15. An anodizing bath for coatingzinc and zinc-base alloy surfaces consisting of an aqueous solution consisting essentially of silicate anion calculated as SiO in an amount of from 0.2 to 0.4 mole per liter, and at least one member selected from the group consisting of chromate anion calculated as CrO vanadate, molybdate and tun gstate anion'mixtures thereof in an amount of from 0.1 to 0.3 mole per liter, and cations of at least one alkali metal selected from the group consisting of sodium and potassium, said solution having a pH of from about 11 to 12.5.

References Cited UNITED STATES PATENTS 2,723,952 11/1 955 Evangelides 20436 X 3,011,958 12/1961 White 20456 3,248,250 4/1966 Collins 148-6.l6 3,293,158 12/1966 McNeill et a1 204-56 JOHN H. MACK, Primary Examiner.

G. KAPLAN, Assistant Examiner.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No 3 ,335 ,074 August 8 1967 Maurice Morgan Wright et a1 It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below In the heading to the printed specification, lines 5 to 7 strike out "ass ignors to The Consolidated Mining and Smelting Company of Canada Limited, Montreal, Quebec, Canada" and insert instead assignors by mesne ass igments, to International Lead Zinc Research Organization, Inc New York, N. Y. a corporation of New York column 8 line 6 after "vanadate," insert permanganate, line 7 after "anion" insert and Signed and sealed this 24th day of September 1968 (SEAL) Attest:

Edward M. Fletcher, 11'. EDWARD J. BRENNER Attesting Officer Commissioner of Patents 

6. AN ANODIC PROCESS FOR COATING ZINC AND ZINC-BASE ALLOY SURFACES WHICH COMPRISES ELECTROLYSING USING ALTERNATING CURRENT AN AQUEOUS SOLUTION CONTAINING SILICATE ANION CALCULATED AS SIO3 IN AN AMOUNT OF FROM ABOUT 0.2 TO ABOUT 2 MOLES PER LITER, CHROMATE ANIONS CALCULATED AS CRO4 IN AN AMOUNT FROM ABOUT 0.05 TO 1.5 MOLES PER LITER AND CATIONS OF AT LEAST ONE ALKALI METAL SELECTED FROM THE GROUP CONSISTING OF SODIUM AND POTASSIUM, SAID SOLUTION HAVING A PH FROM ABOUT 11 TO 13, USING ELECTRODES HAVING EXPOSED SURFACES FORMED BY A METAL SELECTED FROM ZINC AND ZINC-BASE ALLOYS UNTIL A COATING IS FORMED ON SAID ELECTRODES.
 13. AN ARTICLE OF MANUFACTURE HAVING SURFACES FORMED FROM A METAL SELECTED FROM ZINC AND ZINC-BASE ALLOYS COATED WITH A FILM PRODUCED BY THE PROCESS OF CLAIMS
 6. 14. AN ANODIZING BATH FOR COATING ZINC AND ZINC-BASE ALLOY SURFACES CONSISTING OF AN AQUEOUS SOLUTION CONSISTING ESSENTIALLY OF SILICATE ANION CALCULATED AS SIO2 IN AN AMOUNT OF FROM 0.2 TO ABOUT 2 MOLES PER LITER, AND AT LEAST ONE MEMBER SELECTED FROM THE GROUP CONSISTING OF CHROMATE ANION CALCULATED AS CRO4, VANADATE, PERMANGANATE, MOLYBDATE AND TUNGSTATE ANION AND MIXTURES THEREOF IN AN AMOUNT FROM ABOUT 0.05 TO 1.5 MOLES PER LITER, AND CATIONS OF AT LEAST ONE ALKALI METAL SELECTED FROM THE GROUP CONSISTING OF SODIUM AND POTASSIUM, SAID SOLUTION HAVING A PH OF FROM ABOUT 11 TO
 13. 